1. Field of the Invention
The present invention relates to a method of preparing an oxide superconducting wire and a pressure heat treatment apparatus employed for the method, and more particularly, it relates to a method of preparing an oxide superconducting wire capable of preventing the wire from expansion in sintering and a pressure heat treatment apparatus employed for the method.
2. Description of the Prior Art
Generally known is a method of preparing an oxide superconducting wire by heat treating a wire obtained by charging a metal tube with raw material powder for an oxide superconductor and thereafter drawing or rolling the metal tube for sintering the raw material powder for the oxide superconductor. In the aforementioned heat treatment step for sintering, however, the wire is disadvantageously expanded to reduce superconductivity of the obtained oxide superconducting wire.
Japanese Patent Laying-Open No. 5-101723 (1993) proposes a method of preparing an oxide superconducting wire by heat treating a metal tube charged with powder of an oxide superconductor or a flat body thereof under a pressurized atmosphere for sintering the powder of the oxide superconductor. This gazette describes that a wire having excellent superconductivity is obtained by pressure heat treatment.
More specifically, the metal tube charged with the powder of the oxide superconductor is stored in a heat-resistant and pressure-resistant closed vessel to be prevented from expansion in sintering due to the internal pressure increased following temperature increase in the closed vessel. The internal pressure can be obtained from a state equation of gas or the like, and the aforementioned gazette describes that an internal pressure of about 4 atm. can be obtained with a heating temperature of about 900xc2x0 C., for example.
However, the internal pressure obtained following temperature increase in the closed vessel is only about 4 atm. (0.4 MPa), and it is difficult to sufficiently suppress expansion of the metal tube in sintering.
Further, the internal pressure varies with the temperature in the closed vessel, leading to pressure reduction in the process of temperature increase up to the sintering temperature and in the process of temperature reduction to the room temperature after sintering. Therefore, expansion caused by gas generated at a temperature below the sintering temperature cannot be effectively prevented.
An object of the present invention is to provide a method of preparing an oxide superconducting wire capable of effectively preventing expansion in a heat treatment step for sintering raw material powder for an oxide superconductor coated with a metal and obtaining an oxide superconducting wire having desired superconductivity and a pressure heat treatment apparatus employed for this method.
The method of preparing an oxide superconducting wire according to the present invention comprises steps of preparing a wire by coating with a metal raw material powder for a Bixe2x80x94Pbxe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O based oxide superconductor containing bismuth, lead, strontium, calcium and copper and including a 2223 phase having composition ratios of (bismuth and lead), strontium, calcium and copper approximately expressed as 2:2:2:3 and heat treating the wire in a pressurized atmosphere containing oxygen in a prescribed partial pressure, and the total pressure of the pressurized atmosphere is at least 0.5 MPa.
When the total pressure of the pressurized atmosphere is set to at least 0.5 MPa as described above, the wire can be inhibited from expansion caused by gas generated in the raw material powder for the oxide superconductor present in the wire.
In the aforementioned method of preparing an oxide superconducting wire, the total pressure of the pressurized atmosphere is preferably kept at least 0.5 MPa from beginning to end of the heat treatment in the step of heat treating the wire.
The wire can be inhibited also from expansion caused by gas generated in the process of temperature increase and in the process of temperature reduction by keeping the total pressure of the pressurized atmosphere at least 0.5 MPa from the beginning of the process of temperature increase to the end of the process of temperature reduction.
In the aforementioned method of preparing an oxide superconducting wire, the oxygen partial pressure in the pressurized atmosphere is preferably at least 0.003 MPa and not more than 0.02 MPa. Further, the heat treatment temperature is preferably at least 800xc2x0 C. and not more than 840xc2x0 C., and more preferably at least 810xc2x0 C. and not more than 830xc2x0 C. in the step of heat treating the wire.
Superconductivity of the oxide superconducting wire such as the critical current can be improved by defining the oxygen partial pressure and/or the heat treatment temperature as described above.
The method of preparing an oxide superconducting wire according to the present invention preferably further comprises a step of preparing the raw material powder for the oxide superconductor by repeating pulverization and heat treatment.
Further, the method of preparing an oxide superconducting wire according to the present invention preferably further comprises a step of heat treating the raw material powder for the oxide superconductor under decompression and thereafter charging the raw material powder into a metal tube.
In the method of preparing an oxide superconducting wire according to the present invention, the step of preparing the wire preferably includes an operation of drawing the metal tube thereby preparing a wire coated with a metal. The step of preparing the wire preferably includes an operation of charging into another metal tube a plurality of wires obtained by drawing the metal tube and thereafter performing drawing and rolling on this metal tube thereby preparing a tape-like wire.
The method of preparing an oxide superconducting wire according to the present invention is preferably applied to preparation of a wire consisting of a Bixe2x80x94Pbxe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O based oxide superconductor, particularly optimum when employing raw material powder for an oxide superconductor having composition ratios of (Bi+Pb), Sr, Ca and Cu approximately expressed as 2:2:2:3, and suitable for preparing a wire of a Bi-based oxide superconductor including a 2223 phase having the aforementioned composition ratios, for example.
The pressure heat treatment apparatus employed for the method of preparing an oxide superconducting wire according to the present invention comprises a pressure heat treatment furnace storing a target for heat treating the target in a pressurized atmosphere, a pressure measuring device for measuring the total pressure in the pressure heat treatment furnace, an oxygen concentration measuring device for measuring the oxygen concentration in the pressure heat treatment furnace and an oxygen partial pressure control part for controlling the oxygen partial pressure in the pressure heat treatment furnace in response to the total pressure measured by the pressure measuring device and the oxygen concentration measured by the oxygen concentration measuring device.
In the pressure heat treatment apparatus having the aforementioned structure, the oxygen partial pressure in the pressure heat treatment furnace can be precisely controlled. When this pressure heat treatment apparatus is employed for the method of preparing an oxide superconducting wire according to the present invention, the oxygen partial pressure in the pressurized atmosphere can be precisely controlled in the step of heat treating the wire, thereby readily obtaining an oxide superconducting wire having superconductivity such as a desired critical current.
The pressure heat treatment apparatus according to the present invention preferably further comprises a gas introduction device for introducing oxygen gas or non-oxygen gas into the pressure heat treatment furnace in response to a control signal output from the oxygen partial pressure control part.
In this case, the oxygen partial pressure in the pressure heat treatment furnace can be readily controlled by simply introducing oxygen gas or non-oxygen gas into the pressure heat treatment furnace.
According to the inventive method of preparing an oxide superconducting wire, as hereinabove described, the wire can be effectively inhibited from expansion in the heat treatment step, while superconductivity such as the critical current can be improved by controlling the oxygen partial pressure in the pressurized atmosphere within a prescribed range. In the pressure heat treatment apparatus according to the present invention, further, the oxygen partial pressure in the pressure heat treatment furnace can be precisely controlled. When this pressure heat treatment apparatus is employed for the method of preparing an oxide superconducting wire according to the present invention, the oxygen partial pressure in the pressurized atmosphere can be precisely controlled in the step of heat treating the wire, thereby readily obtaining an oxide superconducting wire having superconductivity such as a desired critical current as a result.